feat(db): Add schema migration v6 for merge request support

Introduces comprehensive database schema for merge request ingestion
(CP2), designed with forward compatibility for future features.

New tables:
- merge_requests: Core MR metadata with draft status, branch info,
  detailed_merge_status (modern API field), and sync health telemetry
  columns for debuggability
- mr_labels: Junction table linking MRs to shared labels table
- mr_assignees: MR assignee usernames (same pattern as issues)
- mr_reviewers: MR-specific reviewer tracking (not applicable to issues)

Additional indexes:
- discussions: Add merge_request_id and resolved status indexes
- notes: Add composite indexes for DiffNote file/line queries

DiffNote position enhancements:
- position_type: 'text' | 'image' | 'file' for diff comment semantics
- position_line_range_start/end: Multi-line comment range support
- position_base_sha/start_sha/head_sha: Commit context for diff notes

The schema captures CP3-ready fields (head_sha, references_short/full,
SHA triplet) at zero additional API cost, preparing for file-context
and cross-project reference features.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/006_merge_requests.sql

@@ -0,0 +1,97 @@
+-- Migration 006: Merge Requests, MR Labels, Assignees, Reviewers
+-- Schema version: 6
+-- Adds CP2 MR ingestion support
+
+-- Merge requests table
+CREATE TABLE merge_requests (
+  id INTEGER PRIMARY KEY,
+  gitlab_id INTEGER UNIQUE NOT NULL,
+  project_id INTEGER NOT NULL REFERENCES projects(id),
+  iid INTEGER NOT NULL,
+  title TEXT,
+  description TEXT,
+  state TEXT,                         -- 'opened' | 'merged' | 'closed' | 'locked'
+  draft INTEGER NOT NULL DEFAULT 0,   -- 0/1 (SQLite boolean) - work-in-progress status
+  author_username TEXT,
+  source_branch TEXT,
+  target_branch TEXT,
+  head_sha TEXT,                      -- Current commit SHA at head of source branch (CP3-ready)
+  references_short TEXT,              -- Short reference e.g. "!123" (CP3-ready for display)
+  references_full TEXT,               -- Full reference e.g. "group/project!123" (CP3-ready for cross-project)
+  detailed_merge_status TEXT,         -- preferred, non-deprecated (replaces merge_status)
+  merge_user_username TEXT,           -- preferred over deprecated merged_by
+  created_at INTEGER,                 -- ms epoch UTC
+  updated_at INTEGER,                 -- ms epoch UTC
+  merged_at INTEGER,                  -- ms epoch UTC (NULL if not merged)
+  closed_at INTEGER,                  -- ms epoch UTC (NULL if not closed)
+  last_seen_at INTEGER NOT NULL,      -- ms epoch UTC, updated on every upsert
+  -- Prevents re-fetching discussions on cursor rewind / reruns unless MR changed.
+  discussions_synced_for_updated_at INTEGER,
+  -- Sync health telemetry for debuggability
+  discussions_sync_last_attempt_at INTEGER,  -- ms epoch UTC of last sync attempt
+  discussions_sync_attempts INTEGER DEFAULT 0,  -- count of sync attempts for this MR version
+  discussions_sync_last_error TEXT,           -- last error message if sync failed
+  web_url TEXT,
+  raw_payload_id INTEGER REFERENCES raw_payloads(id)
+);
+
+CREATE INDEX idx_mrs_project_updated ON merge_requests(project_id, updated_at);
+CREATE INDEX idx_mrs_author ON merge_requests(author_username);
+CREATE INDEX idx_mrs_target_branch ON merge_requests(project_id, target_branch);
+CREATE INDEX idx_mrs_source_branch ON merge_requests(project_id, source_branch);
+CREATE INDEX idx_mrs_state ON merge_requests(project_id, state);
+CREATE INDEX idx_mrs_detailed_merge_status ON merge_requests(project_id, detailed_merge_status);
+CREATE INDEX idx_mrs_draft ON merge_requests(project_id, draft);
+CREATE INDEX idx_mrs_discussions_sync ON merge_requests(project_id, discussions_synced_for_updated_at);
+CREATE UNIQUE INDEX uq_mrs_project_iid ON merge_requests(project_id, iid);
+
+-- MR-Label junction (reuses labels table from CP1)
+CREATE TABLE mr_labels (
+  merge_request_id INTEGER REFERENCES merge_requests(id) ON DELETE CASCADE,
+  label_id INTEGER REFERENCES labels(id) ON DELETE CASCADE,
+  PRIMARY KEY(merge_request_id, label_id)
+);
+CREATE INDEX idx_mr_labels_label ON mr_labels(label_id);
+
+-- MR assignees (same pattern as issue_assignees)
+CREATE TABLE mr_assignees (
+  merge_request_id INTEGER REFERENCES merge_requests(id) ON DELETE CASCADE,
+  username TEXT NOT NULL,
+  PRIMARY KEY(merge_request_id, username)
+);
+CREATE INDEX idx_mr_assignees_username ON mr_assignees(username);
+
+-- MR reviewers (MR-specific, not applicable to issues)
+CREATE TABLE mr_reviewers (
+  merge_request_id INTEGER REFERENCES merge_requests(id) ON DELETE CASCADE,
+  username TEXT NOT NULL,
+  PRIMARY KEY(merge_request_id, username)
+);
+CREATE INDEX idx_mr_reviewers_username ON mr_reviewers(username);
+
+-- Add FK constraint to discussions table for merge_request_id
+-- Note: SQLite doesn't support ADD CONSTRAINT, the FK was defined in CP1 but nullable
+-- We just need to add an index if not already present
+CREATE INDEX IF NOT EXISTS idx_discussions_mr_id ON discussions(merge_request_id);
+CREATE INDEX IF NOT EXISTS idx_discussions_mr_resolved ON discussions(merge_request_id, resolved, resolvable);
+
+-- Additional indexes for DiffNote queries (notes table from CP1)
+-- These composite indexes enable efficient file-context queries for CP3
+CREATE INDEX IF NOT EXISTS idx_notes_type ON notes(note_type);
+CREATE INDEX IF NOT EXISTS idx_notes_new_path ON notes(position_new_path);
+CREATE INDEX IF NOT EXISTS idx_notes_new_path_line ON notes(position_new_path, position_new_line);
+CREATE INDEX IF NOT EXISTS idx_notes_old_path_line ON notes(position_old_path, position_old_line);
+
+-- CP2: capture richer diff note position shapes (minimal, still MVP)
+-- These fields support modern GitLab diff note semantics without full diff reconstruction
+ALTER TABLE notes ADD COLUMN position_type TEXT;           -- 'text' | 'image' | 'file'
+ALTER TABLE notes ADD COLUMN position_line_range_start INTEGER;  -- multi-line comment start
+ALTER TABLE notes ADD COLUMN position_line_range_end INTEGER;    -- multi-line comment end
+-- DiffNote SHA triplet for commit context (CP3-ready, zero extra API cost)
+ALTER TABLE notes ADD COLUMN position_base_sha TEXT;       -- Base commit SHA for diff
+ALTER TABLE notes ADD COLUMN position_start_sha TEXT;      -- Start commit SHA for diff
+ALTER TABLE notes ADD COLUMN position_head_sha TEXT;       -- Head commit SHA for diff
+
+-- Update schema version
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (6, strftime('%s', 'now') * 1000, 'Merge requests, MR labels, assignees, reviewers');